When a project is finished and blocked, does it need to be re-blocked after each laundering, or will the project keep its size and shape from the original blocking?
If the project is acrylic, you don't really need to block at all, just wash and dry as the ballband directs. For wool, I've found that blocking once is enough. I shape the piece a bit as it dries after subsequent washes but it doesn't need to be pinned out again.
Acrylic items do benefit from a light steaming - this relaxes the stitches (pull stitches into shape and pin garment BEFORE steaming) and helps the fabric to feel better against the skin. If your project is acrylic, it would not harm to give it another light steaming every now and then if it is starting to sag under the pressures of frequent use.
As for wool and other fibres, I would go with what salmonmac has advised. Just be sure to wash according to the ball band instructions, so that your item can keep its shape and texture (so that it will not need another blocking).